The present invention relates to a filter integrated circuit suitable for integration of a filter into a monolithic IC formed on a silicon wafer or the like.
As lowpass, highpass and bandpass filters or phase equalizers for producing desired signals in conventional electronic circuits, block filters having discrete components of inductance L, capacitance C and resistance R have been widely used. As more and more electronic circuits are realized as integrated circuits (monolithic IC's), these filters become obstacles to cost reduction and reduction of size and weight of electronic circuits. Especially for portable devices in which mobility is an important factor, reduction in size and weight is important and it is demanded to realize filters in the form of integrated circuits.
Since it is difficult to realize the inductance L as an integrated circuit, filters suitable for integration are active filters which can be formed by using only the capacitance C and resistance R. For example, twin-T-shaped trap filters can be formed by using only capacitors and resistors. The trap frequency fr of such a filter is represented as EQU fr=1/2.pi.CaRa
where Ca is the capacitance of the capacitor used in the twin-T-shaped trap filter and Ra is the resistance of the resistor used in that filter.
When such a trap filter is to be integrated, deviations in the capacitance value and the resistance value pose a problem. That is to say, the values of the capacitance and resistance in the IC are affected by deviations in the impurity concentrations and mask alignment etc. For example, the absolute value of the capacitance varies by .+-.10 to 15% and the absolute value of the resistance varies by .+-.10%. These deviations are rather large values. In the above described example, therefore, the trap frequency of the integrated twin-T-shaped trap filter also varies by .+-.20 to 25% in the worst case, the practical use being extremely difficult. In accordance with a countermeasure disclosed in Japanese Patent Publication No. 58083/82, the resistance value of the resistor located on the IC chip is changed by using the laser trimming to correct the deviation. Although this countermeasure has been used, many problems still remain with respect ot the precision and the yield rate.
In variable attenuation circuits disclosed in Japanese Patent Publication No. 58083/82 and U.S. Pat. No. 3,761,741, the fact that the emitter resistance of the transistor is varied by a change in the DC current is used. It is known that the variation in the filter characteristics caused by deviations of element values of the IC can be corrected by using the similar technique. However, it is difficult to apply this technique to all filters including trap filters. In addition, deviations of elements of the IC must be corrected by external adjustment, resulting in a higher cost.